To solve a problem that priming in a pump casing is discharged by siphon phenomenon and necessary quantity of priming is missing in pump restart in conventional self-priming pumps.

In this self-priming pump, a first casing provided with a suction port and a delivery port on an outer hull and having a suction channel and a delivery channel isolated by providing a bulkhead inside is formed in such a manner that inner volume of the suction channel is larger than that of the delivery channel.